This is a continuation of studies of the pathophysiology of the circulation of the distal segment of the optic nerve and the choroid. The tissue oxygen tension in the prelaminar optic disk and in the outer retina and the fovea centralis will be measured polarographically. The effect of changes in the systemic blood pressure, sympathetic nerve stimulation, and sympathectomy will be examined. The blood flow in the prelaminar optic disk will be measured by the method of hydrogen clearance. The effect on optic disk blood flow of changes in the perfusion pressure by changing the intraocular pressure and by changing the systemic blood pressure will be studied. The effect on the optic disk blood flow of sympathetic stimulation, and sympathectomy will be examined. The blood flow in the choroid will be measured by the method of indocyanine green clearance. The effect on choroidal blood flow of changes in the systemic blood pressure, sympathetic nerve stimulation, and sympathectomy will be examined. The effect on permeability of the choroicapillaris of systolic hypertension in sympathectomized animals will be studied by fluorescein angiography and light and electron microscopy.